The present invention generally concerns composite materials formed from a superconducting oxide and a metal. The present invention more particularly concerns a process for fabricating high temperature superconductor/metal composite materials as well as the resultant materials.
As used herein, the term "critical temperature" or "T.sub.c " refers to that temperature below which the Meissner effect is manifest. The Meissner effect is the expulsion of magnetic flux that occurs when a material is cooled below its superconducting transition. It is considered absolute proof of superconductivity.
Superconductivity, particularly high T.sub.c superconductivity, is the subject of considerable research efforts. Research programs focus upon, among other considerations, means to increase the superconducting transition temperature, understanding reasons and mechanisms for superconductive properties of certain materials and development of new superconducting materials.
A practical limitation on the usefulness of superconducting oxide materials is their lack of ready conversion to fabricated products with mechanical integrity as well as stable superconducting properties. Literature references addressing this limitation consistently maintain that formation of intimate composites of superconductors and metals is essential to use of the superconductors in a variety of applications.
In an article entitled "Review on the Fabrication Techniques of A-15 Superconductors", Cryogenics, Vol. 27, No. 7, pages 361-78 (1987), R. G. Sharma describes various processes used to prepare wires of brittle A-15 superconductors such as Nb.sub.3 Sn and V.sub.3 Ga for use in high field magnetic coils. All of the processes lead to wires in which filaments of superconductor one to ten micrometers are embedded intimately in a metal matrix composed of metals such as copper and bronze. Such a structure is essential to provide stability against disturbances of a electrical, thermal or mechanical nature in the finished part. In addition, metals such as niobium, bronze, tin or copper are used as precursors for the superconductor filaments in order to aid in their fabrication.
In a review article entitled "Superconducting Coils", The Science and Technology of Superconductivity, Volume 2, pages 497-538 (1973), Z. J. J. Stekly describes the importance of having metals in intimate contact with the superconductor within a fabricated coil for both ductile and brittle superconductors, including Nb.sub.3 Sn, NbTi and NbZr. He suggests that criteria for stability of the fabricated coil include small superconductor size and the addition of a conductor with high electrical and thermal conductivity. He provides examples of conducting strands, including NbTi filaments having a diameter of 50 to 100 micrometers.
Braginski U.S. Pat. No. 4,411,959 disclose a superconducting composite wire comprising an encapsulating sheath of ductile conducting metal and encapsulated filaments of essentially contiguous submicron particle superconducting powder. The superconducting powder may be a niobium or vanadium compound such as Nb.sub.3 Ga or V.sub.3 Ga or a Chevrel phase such as PbMo.sub.6 S.sub.8. The Powder contains up to 10% by volume of a lubricant.
Dubots U.S. Pat. No. 4,594,218 disclose a multi-step process for making lengths of superconductor from a ternary chalcogenide of molybdenum. In step one, a powder of a chalcogenide of molybdenum is mixed with a powder of a smaller particle size. The powder is chosen from constituents of the chalcogenide, aluminum, silver, gallium, rhenium and titanium. The powder mixture is then sheathed with a metal wall formed from molybdenum, niobium, tantalum, titanium or vanadium. The sheathed mixture is then drawn and cold worked by conventional cable-making techniques to form superconducting lengths. Finally, the superconducting length is heated to a temperature of about 800.degree. Celsius for a period of at least twenty hours.
Roy U.S. Pat. No. 3,752,665 disclose synthesis of superconducting intermetallic compounds by explosive compaction of a stoichiometric powder mixture of the constituent metallic elements. One such compound is Nb.sub.3 Sn.
Winter U.S. Pat. No. 4,050,147 incorporate fine superconducting particles with a medium diameter of up to 500 Angstroms into a ductile metallic matrix by one of three different methods. The superconducting particles are compounds of niobium or vanadium such as compounds of A.sub.3 B with BW, where A is Nb or V and B is Al, Ge, Si, Ga or Sn. One method includes mixing a powdered compound such as Nb.sub.3 Sn with a metal powder like copper, compacting the mixed powder, and sheathing the compacted powder in a container of the same metal. The container is evacuated and closed before it is extruded and further drawn into wires.
Cannon U.S. Pat. No. 3,301,643 produce superconducting composite materials comprising a zeolite matrix having atomic size filamentary pores extending throughout and filled with a material capable of being rendered superconductive. One means of preparing such a material involves infusion of molten metal under pressure into a zeolite matrix. An alternate means includes steps of impregnating zeolite particles with a solvent solution containing metal ions and thereafter removing the solvent and reducing the metal ions to free metal. The impregnated particles may then be formed into a composite body of appropriate shape.
DT 2,646,096 discloses preparation of ductile superconducting wire or strip by dispersing superconducting particles in conventional conductor matrix and extruding the resulting coarse granulate. In one example, 10% NbN powder and 90% pure Al powder are milled for 200 hours to provide a granulate with 300 micrometer particle size. The granules are cold pressed into bars then heated and extruded into rods which are subsequently made into wire.
Rosi et al. (DT 1,490,242) disclose superconductors made from two pulverized metal components. One component may be a sintered compound of two superconducting metals, the other either a super- or a normal conducting metal. The two components are pressed together, e.g., with a pressure of 560 kg/cm.sup.2 and then annealed in a vacuum, e.g., at 700.degree. C. for two hours.
The teachings of each of the foregoing references focus upon fabricating composites of normal metals with low T.sub.c nonoxide superconductors. These materials, which include such commercial materials as NbTi and Nb.sub.3 Sn, have an inherent, fundamental limitation in that they have a maximum T.sub.c of 23.5K.
Recent discoveries of a number of oxide superconductors with T.sub.c 's as high as 125K provide certain advantages over the low T.sub.c nonoxide superconductors. They can, for example, be cooled more cheaply than the low T.sub.c superconductors. Notwithstanding such advantages, the oxide superconductors also possess at least one shortcoming that poses a formidable challenge to preparing composites of the oxide superconductors with normal metals. That shortcoming is the generally high degree of reactivity between these metals and the superconducting oxides. This reactivity suggests that fabrication of composites or mixtures of high T.sub.c superconducting oxides and normal metals is not a simple extension of the aforementioned techniques used with low T.sub.c nonoxide superconductors. The noble metals, e.g., silver, gold, platinum and palladium, are often suggested as a solution to the reactivity problem.
Yurek U.S. Pat. No. 4,826,808 disclose a superconducting oxide-metal composite in which a noble metal phase is intimately mixed with a superconducting oxide phase to achieve desired mechanical properties. They classify a metal as noble if its oxide is thermodynamically unstable under the reaction conditions employed relative to the superconducting oxide that forms. The noble metal may be a metallic element such as gold, platinum, palladium or silver. The noble metal may also be an excess amount (stoichiometrically) of one of the metallic elements of the oxide, e.g., copper. The composite may, for example, be prepared by alloying the metallic elements of the superconducting oxide with the noble metal and thereafter oxidizing the alloy under conditions sufficient to oxidize the superconducting oxide components but not the noble metal.
Gazit et al., in an article entitled "Preparation of High Temperature Superconductor-Metal Wire Composites", Materials Research Bulletin, Vol. 24, pages 467-74 (1989), disclose preparation of composite wires by pulling platinum wires through a melt composed of bismuth superconductors. The superconductors are Bi.sub.1.8 Sr.sub.1.8 Ca.sub.1.2 Cu.sub.2.2 O.sub.8 and Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8. Although these wires are not superconducting, the authors suggest that gold, rhodium or iridium wires should be.
Deslandes et al., in an article entitled "Research of the Effective Role of Silver Additions to YBa.sub.2 Cu.sub.3 O.sub.7 ", Solid State Communications, Vol. 71, No. 5, pages 407-10 (1989), teach that metallic silver additions improve the current carrying capacity of fabricated ceramics by cleaning the grain surfaces.
S. Jin et al., in an article entitled "Superconductivity in the Bi-Sr-Ca-Cu-O Compounds with Noble Metal Additions", Applied Physics Letters, Vol. 52, No 19, pages 1628-30 (1988), prepare particle composites of Bi.sub.4 Sr.sub.3 Ca.sub.2 Cu.sub.4 O.sub.16 with gold, silver or platinum by sintering. Only the silver-containing composites are benign. Both gold and platinum-group metals significantly suppress or eliminate the superconducting properties of the fabricated composite.
S. X. Dou et al., in an article entitled "Superconductivity in an Ag-doped Bi-Pb-Sr-Ca-Cu-O System", Applied Physics Letters. Vol. 56, No 5, pages 493-94 (1990), describe conditions under which successful particle composites of the title composition can be fabricated by sintering.
Although some success in fabricating metal-superconducting oxide composites is evident from the preceding references, at least two shortcomings remain. First, noble metals are expensive and may cause the resulting composites to be prohibitively expensive. Second, noble metals such as silver and gold are mechanically soft and may not be suitable for all applications. For these reasons, among others, there is considerable interest in encapsulating superconducting oxides in non-noble or base metals such as copper and aluminum.
Two methods are commonly used in an effort to encapsulate superconducting oxides in base metals. One method involves placing the superconducting oxide into a tube formed of a base metal and thereafter fabricating the tube by various techniques such as wire drawing. This method is generically referred to as the "powder in tube" method. The other method includes a step of cladding an already formed superconducting article with a layer of base metal.
T. J. Richardson et al., in an article entitled "Aluminum Cladding of High T.sub.c Superconductors by Thermocompression Bonding", Applied Physics Letters, Vol. 53, No.23, pages 2342-43 (1988), disclose the cladding of high T.sub.c oxide superconducting ceramics with aluminum. A two micrometer layer of silver is applied to the ceramic before cladding to reduce reactivity between the superconductor and the aluminum.
Min-Seok Oh et al., in an article entitled "Fabrication and Microstructure of Composite Metal-Clad Ceramic Superconducting Wire", Journal American Ceramic Society, Vol. 72, No. 11, pages 2142-47 (1989), disclose the use of the "powder in tube" method where the tube is either pure silver or a thin inner layer of silver covered by a thick wall of stainless steel or nickel. Although the thin inner layer is designed to reduce reaction between YBa.sub.2 Cu.sub.3 O.sub.7 and the base metal, the authors report that the effort is largely unsuccessful. They find considerable evidence of a reaction which destroys most, if not all, of the superconductivity.
D. Shi et al., in an article entitled "Swagged Superconducting Wires", Materials Letters, Vol. 7, No. 12, pages 428-32 (1989), disclose a "powder in tube" method where the tube is made of copper. The copper tube is, however, removed prior to fabricating a wire in an effort to reduce reactivity between the copper and the superconducting oxide. As such, the final fabricated article is no longer a composite.
L. E. Murr et al., in an article entitled "Introducing: The Metal-Matrix High-Temperature Superconductor", Advanced Materials & Processes inc. Metal Progress 10/87, pages 36-44, teach that solid cylindrical composites may be prepared by incorporating mixtures of copper powder and cuprate superconducting powders in an arrangement of copper tubes via explosive compaction. The resulting monoliths could be rolled, drawn, or extruded into wire at relatively low temperature. They suggest that other metals might be substituted for the copper powder.
L. E. Murr et al., in an article entitled "Fabrication of Metal/High-Temperature Superconductor Composites by Shock Compression", SAMPE Journal, Vol. 24, No. 6, pages 15-18 (Nov./Dec. 1988), disclose a combination of explosive welding and explosive powder consolidation to consolidate, bond and encapsulate reactive and temperamental copper oxide-based, high-T.sub.c superconducting powders in a supporting metal matrix.
The powder in tube technique is, as noted hereinabove, not completely successful due to reactivity between the superconducting oxide and the non-noble or base metal. In addition, the degree of intimacy of contact between the superconductor and the metal is limited to the interface between the surface of a metal layer and the outer surface of the body of powder. In other words, contact between the superconductor and the metal occurs only on a macroscopic level. By way of contrast, contact between the superconductor and the metal in low T.sub.c superconductor composites is much more intimate. The latter composites typically consist of one to ten micrometer filaments of superconductor dispersed in a metal phase. If such a procedure were used with the superconducting oxides, an even greater level of reactivity would be expected.
Notwithstanding an expectation of greater reactivity, some efforts are directed toward duplicating the low T.sub.c composite work for high T.sub.c superconducting oxides.
I. Chen et al., in Superconductivity News, pages 15-16 (June 1988), disclose aluminum/silver/123 composites prepared by a conventional powder metallurgy process. They suggest that superconducting materials other than 123 (YBa.sub.2 Cu.sub.3 O.sub.7-X) will also work.
A. Goyal et al., in an article entitled "Cermets of the Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-.delta. Superconductors", Materials Letters, Vol. 6, No. 8, 9 (May 1988), describe cermets prepared from intimate mixtures of copper, nickel or tin (15 weight percent of each) with a YBa.sub.2 Cu.sub.3 O.sub.7 superconductor. They suggest that copper and nickel composites are superconducting because of zero resistivity. They do not, however, comment upon the presence or absence of reactivity between the metal and the superconducting oxide.
R. C. Chan et al., in a talk entitled "Superconducting Pastes and Their Applications", American Ceramic Society, Annual Meeting, Indianapolis, Ind., Apr. 27, 1989, teach that composites can also be formed by infiltrating liquid tin into porous superconductor preforms. They suggest that the superconducting properties are not detrimentally affected by this technique. This method is, however, limited to low melting base metals due to the general tendency of superconducting oxides to be very reactive with molten base metals. The low melting base metals are usually soft with poor mechanical properties. These properties carry over to the resulting composites.